Ne'er-do-wells can purchase the card numbers in batches of one million at between $20 and $100 per card. A lucrative enterprise for whoever's responsible, indeed.

Not only are victims susceptible to fraudulent purchases, but as Krebs explains, if they used their card as a debit card and entered their pin numbers, their cash is vulnerable as well:

Armed with that information, thieves can effectively clone the cards and use them in stores. If the dumps are from debit cards and the thieves also have access to the PINs for those cards, they can use the cloned cards at ATMs to pull cash out of the victim’s bank account.